1. Field of Technology
The present disclosure relates to methods for limiting the formation of an electrically resistive surface layer or “scale” on stainless steels when the steels are subjected to high-temperature, oxidizing conditions. The present disclosure also relates to stainless steels and articles of manufacture including stainless steels, wherein the steels have a reduced tendency to form electrically resistive scale thereon when the steels are subjected to high-temperature, oxidizing conditions.
2. Description of the Background of the Technology
Fuel cells are energy conversion devices that generate electricity and heat by electrochemically combining a gaseous fuel and an oxidizing gas via an ion-conducting electrolyte. Fuel cells convert chemical energy directly into electrical energy in the absence of combustion, providing significantly higher conversion efficiencies than reciprocating engines, gas turbines, and certain other conventional thermomechanical energy production devices. In addition, for the same power output, fuel cells produce substantially less carbon dioxide emissions than fossil fuel-based power generation technologies. Fuel cells also produce negligible amounts of SOx and NOx, the main constituents of acid rain and photochemical smog.
Several types of fuel cells have been developed, differing primarily in the materials utilized as the fuel cell electrolyte. NASA originally developed alkaline fuel cells including a liquid electrolyte in the 1960's to power Apollo and other spacecraft. Liquid electrolytes, however, typically are corrosive and can be difficult to handle. Solid oxide fuel cells (SOFCs), in contrast, are constructed entirely of solid-state materials and employ a fast oxygen ion-conducting ceramic material as the electrolyte. SOFCs operate in a temperature range of about 500° C.-1000° C. to facilitate solid-state transport. The advantages of SOFCs include high energy efficiency and relatively few problems with electrolyte management. SOFCs also produce high-grade waste heat, which can be used in combined heat and power devices, and harnessed for internal reforming of hydrocarbon fuels.
A single SOFC subunit or “cell” includes an anode and a cathode, separated by the electrolyte. During operation of the SOFC cell, an oxidant (such as oxygen or air) is fed into the fuel cell on the cathode side, where it supplies oxygen ions to the electrolyte by accepting electrons from an external circuit through the following half-cell reaction:½O2(g)+2e−→O−2 
The oxygen atoms pass through the ceramic electrolyte via solid state diffusion to the electrolyte/anode interface. The SOFC can employ hydrogen (H2) and/or carbon monoxide (CO) as a basic fuel. Operationally, pure hydrogen can be used as supplied. If a hydrocarbon fuel such as methane, kerosene, or gasoline is used, it must first be partially combusted, or “reformed”, to provide hydrogen and carbon monoxide. This may be accomplished internally within the fuel cell, aided by the high cell operating temperature and by steam injection. The fuel gas mixture penetrates the anode to the anode/electrolyte interface, where it reacts with the oxygen ions from the ion-conducting electrolyte in the following two half-cell reactions:H2(g)+O−2→2e−+H2O(g) CO(g)+O−2→2e−+CO2(g).These reactions release electrons, which re-enter the fuel cell's external circuit. The flow of electrical charge due to oxygen ion transport through the electrolyte from cathode to anode is balanced exactly by the flow of electrical charge through electron conduction in the external circuit. The cell's driving force is the need to maintain overall electrical charge balance. The flow of electrons in the external circuit provides useful power.
To generate a reasonable voltage, fuel cells are not operated as single units, but instead as “stacks” composed of a series arrangement of many individual cells, with an “interconnect” joining and conducting current between the anode and cathode of each of the immediately adjacent cells. A common stack design is the flat-plate or “planar” SOFC (PSOFC), which is shown in a schematic form in FIG. 1. In the PSOFC 10 of FIG. 1, a single energy conversion cell 12 includes a cathode 20 and an anode 30 separated by the electrolyte 40. An interconnect 50 separates the anode 30 from the cathode 60 of an immediately adjacent energy conversion cell 14 (not fully shown) within the stack. Thus, PSOFC 10 includes a repeating arrangement of cells, substantially identical to cell 12, with an interconnect disposed between each adjacent cell.
The interconnects are critical SOFC components and serve several functions, including separating and containing the reactant gases, providing a low resistance current pathway to electrically connect the cells in series, and providing structural support for the stack. The interconnects must be made of a material that can withstand the harsh, high-temperature environment within the cells, must remain suitably electrically conductive throughout the fuel cell's service life, and must have a coefficient of thermal expansion (CTE) that is sufficiently similar that of the cells' ceramic components to ensure that the stack's requisite structural integrity and gas-tightness is maintained. Initial PSOFC designs utilized LaCrO3 ceramic interconnects. LaCrO3 ceramic does not degrade at the high SOFC operating temperatures and has a CTE that substantially matches the other ceramic components of the fuel cell. LaCrO3 ceramic, however, is brittle, difficult to fabricate, and expensive.
To address deficiencies of ceramic electrolytes, interconnects have been made from certain metal alloys. Metallic interconnects are desirable for reasons including their relatively low manufacturing cost, high electrical and thermal conductivities, and ease of fabrication, which aids in the formation of gas channels and allows for a high degree of dimensional control. Alloys proposed for interconnect applications include nickel-base alloys (such as AL 600™ alloy), certain austenitic stainless steels (such as Types 304, 309, 310 and other alloys in the 300 Series family), and certain ferritic stainless steels (such as, for example, E-BRITE® alloy and AL 453™ alloy). Table 1 provides nominal compositions for several of the foregoing commercially available nickel-base and stainless steel alloys, all of which are available from ATI Allegheny Ludlum, Pittsburgh, Pa.
TABLE 1Composition (weight percent)AlloyNiCrFeAlSiMnOtherAL 453 ™ 0.3 max.22bal.0.60.30.3 0.06 Ce + Lamax.E-BRITE ®0.15 max.26bal.0.10.20.051 MoAL 600 ™bal.15.58—0.20.25—Type 304818bal.————
Certain characteristics of ferritic stainless steels including at least about 16 weight percent chromium make them particularly attractive for PSOFC interconnect applications including, for example, low cost, excellent machinability, and CTEs compatible with conventional ceramic electrodes. Ferritic stainless steels including 16-30 weight percent chromium and less than 0.1 weight percent aluminum are believed to be particularly suited for interconnect applications. Specific examples of ferritic stainless steels considered suitable for PSOFC interconnect applications include AISI Types 430, 439, 441, and 444 stainless steels, as well as E-BRITE® alloy. The CTEs of the ceramic electrode material lanthanum strontium manganate and AISI Type 430 ferritic stainless steel, for example, are reported to be about 11-13×10−6 and about 9-12×10−6, respectively.
Ferritic stainless steels, however, commonly include moderate levels of silicon, either as an intentional alloying addition or as a residual from the steelmaking process. Silicon is commonly present in ferritic stainless steels at levels of about 0.3 to 0.6 weight percent. Silicon is not commonly added to ferritic stainless steels as an intentional compositional element, but it may be added during the melting of stainless steels as a process element. A portion of the silicon added to the melt, however, unavoidably makes its way into the steel. Therefore, even though silicon is intentionally added in such cases, it may be considered a residual impurity in the steel.
Silicon is detrimental to the operational efficiency of ferritic stainless steel interconnects since it tends to migrate to the steel surface/scale interface and form a thin, generally continuous, highly electrically resistive SiO2 (silica) layer at the interface. Formation of silica at the interface between the steel and the scale formed on the steel increases the contact electrical resistivity of the interconnects over time. This makes it increasingly difficult for electrons to pass through the interface region between the interconnect and the electrodes, and thereby progressively impairs the ability of the interconnects to conduct current between the cells. This process, over time, can significantly reduce the overall efficiency of SOFCs including ferritic stainless steel interconnects. As such, it is one factor considered when selecting a suitable interconnect material from among the various available ceramic and alloy materials.
Accordingly, it would be advantageous to provide a method for eliminating or reducing the tendency for electrically resistive silica to form on the surface of ferritic stainless steels when the steels are subjected to oxidizing conditions, such as conditions to which SOFC interconnects are subjected.